It has been conventionally known to use a foil bearing as a bearing for a rotor shaft. A foil bearing typically comprises a housing surrounding the rotor shaft and a plurality of foils disposed in a space or gap between the rotor shaft and the housing and arranged in a circumferential direction in such a manner that each of the foils is attached to the housing in a cantilever fashion with its free end being urged toward the rotor shaft. As the rotor shaft rotates, a fluid such as ambient air is drawn in between the rotor shaft and the foils, creating a fluid film between an outer surface of the rotor shaft and the foils to allow the rotor shaft to rotate with low friction. Such a foil bearing that supports a load of the rotor shaft via the fluid film formed as a result of the rotor shaft rotation may be called a hydrodynamic foil bearing.
In the foil bearing as described above, characteristics of the fluid film formed between the rotor shaft and the foils may vary between a state where the rotor shaft rotation speed is low, e.g., at the start up or shut down, and a state where the rotor shaft rotation speed is high. Thus, in order to achieve a stable rotation in both states, it is desired that a preload of the foils against the rotor shaft is adjustable according to the rotation speed of the rotor shaft.
U.S. Pat. No. 4,445,792 issued to Trippett has disclosed a foil bearing in that the preload of the foils can be adjusted. The foil bearing has a housing surrounding the rotor shaft, and a plurality of foil mounts arranged along the circumferential surface of the rotor shaft and rotatably supported relative to the housing, wherein each mount has a foil attached thereto to cantilever the foils and has a driven portion. Further, a drive mechanism (ring gear) rotatable relative to the housing is provided outside the housing so as to be engageable with the driven portions of the foil mounts to simultaneously rotate the mounts and vary the preload of the foils against the shaft surface.
In such a foil bearing, however, the preload of the foils against the rotor shaft relies upon an elasticity of each foil, and thus it is difficult to vary the preload in a sufficiently large range, and further, an excessive force tends to concentrate on a root portion of each cantilevered foil.
Other known prior art may include U.S. Pat. No. 3,893,733 issued to Silver et al, which has disclosed to use foil supports that slidingly contact the foils to increase the stiffness (or rigidity) of the foils. U.S. Pat. No. 4,178,046 issued to Silver et al has disclosed a foil bearing in that each foil is mounted intermediate the ends thereof. U.S. Pat. No. 4,128,280 issued to Purtschert has disclosed various types of floating gas bearing utilizing a magnet in the bearing. Further, Japanese Patent Application Laid-Open (kokai) No. 10-292818 has disclosed to use a thrust magnetic bearing to assist a hydrodynamic gas bearing at low rotational speed of the rotor shaft.